The invention relates to the field of motorvehicle safety and relates more particularly to pyrotechnic gas generators for inflating airbags.
New generators, called adaptive generators, have recently appeared so as to limit the risk of injuries to the body which are caused by the airbag being deployed under exceptional conditions. These generators, combined with suitable detection systems, allow the gas flow rate to be modulated depending on various parameters such as, for example, the ambient temperature, the nature and intensity of the impact, and the morphology and position of the passenger. In order to construct "adaptive" generators, it is possible to use, on the one hand, hybrid-type generators, i.e. generators whose gas intended to inflate the airbag consists of a mixture of the gas coming from combustion of the pyrotechnic charge and the gas stored in a pressurized container, and on the other hand, generators whose single combustion of the pyrotechnic charge or charges allows the airbag to be deployed. U.S. Pat. No. 3,972,545 describes a "passenger" generator which comprises two combustion chambers each one provided with a pyrotechnic charge and with an initiator, the two chambers being separated by a consumable partition. Depending on the intensity of the impact, the actuation of the two initiators connected to a single power supply can be staggered. When a low-voltage electrical signal is emitted, only one chamber is ignited by means of the initiator which it contains, the second chamber being ignited when the partition separating the two chambers has been consumed. On the other hand, when a high-voltage signal is emitted, the two initiators are actuated simultaneously, thus resulting in simultaneous ignition of the two combustion chambers. It therefore follows that the volume of gas released into the airbag is constant and inflates it to the maximum amount under all circumstances.
Patent Application EP 609,981 defines a "driver" gas generator which makes it possible to vary the time of combustion of the pyrotechnic charge by means of two initiators which can be fired successively or simultaneously. However, on the one hand, the use of two independent and separate initiators also requires separate power supplies, thus increasing the cost and the amount of space required, and, on the other hand, the volume of gas generated is constant under all circumstances.
Patent Application EP 0,382,552 discloses a generator more especially designed to protect a passenger. This generator consists of a monobloc cylindrical piece divided into two parts by an internal partition and has two combustion chambers, each one containing a pyrotechnic charge and an initiator, each initiator being connected to a separate power supply. In operation, the two chambers are ignited at staggered times so as to optimize the gas flow rate and therefore the pressure level obtained in the airbag.
Thus, these generators allowing the rate of inflation of the airbag to be modulated cannot, depending on the circumstances, reduce the final volume of gas released into the airbag. Moreover, the relatively complex process for manufacturing these generators and, where appropriate, the use of separate power supplies make it difficult to transfer such technologies for making compact low-cost generators.
Those skilled in the art are therefore looking for a pyrotechnic gas generator which is easy and inexpensive to manufacture and makes it possible, depending on the situation, either to release only a certain volume of gas and therefore to inflate the airbag only partially, or to release the maximum volume of gas so as to inflate the airbag fully, with the possibility of adjusting the rate of inflation.